Factors influencing T c in 123 copper-oxide superconductors

Cosubstitution of Ca ²⁺ for Y ³⁺ and La ³⁺ for Ba ²⁺ in Y _{1- z}Ca _zBa _{2-2 y}La _{2 y}Cu ₃O _{6+ x} has demonstrated that, in this system, there is a critical oxygen concentration x_c=0.96 + 0.5 n_c, where n=2 y- z, such that disordering of the oxygen in the Cu(1)O _x planes has no effect on T_c for x< x_c, but creates mobile-hole traps for x> x_c. Morever, substitution of La ³⁺ for Ba ²⁺ in the Ba _{1- y}La _yO layers creates local disorder of the oxygen neighboring a La ³⁺ ion, which reduces the orthorhombicity of the structure. These two observations explain why the systems Y _{1- z}Ca _zBa _{2- z}La _zCu ₃O _{6+ x} and LaBa _{2-2 y}Ca _{2 y}Cu ₃O _{6+ x}=La _{1- z}Ca _zBa _{2- z}La _zCu ₃O _{6+ x} maintain a T_c > 80 ( ) K through an orthorhombic-tetragonal transition with increasing z whereas LaBa _{2-2 y}Sr _{2 y}Cu ₃O _{6+ x}=La _{1- z}Sr _z(Ba _{2-2 y}Sr _{2 y- z}La _z)Cu ₃O _{6+ x} exhibits a T_c that decreases monotonically with inc reasing y. They demonstrate that T_c is independent of the space-group symmetry where hole trapping does not occur. Decreasing the lattice a-parameter and/or the basic character of the BaO basal “planes” degrades T_c in a relatively minor manner. These findings also implicate c-axis oxygen in any oxygen disordering of the BaO-Cu(1)O _x-BaO intergrowth layer.